Not so inhospitable : It used to be unthinkable that life could exist at
temperatures near boiling, but some intrepid archaeans thrive under these
conditions. Geysers, like the one shown above, are home to such microbes
and may help us understand how life existed when the Earth was young.

The search for fossils of Archaea faces a number of problems. First of all,
they're very tiny organisms and so will leave microscopic fossils. Any search
for fossilized archaeal cells would require a lot of time spent with a
microscope and a lot of patience. In fact, there are fossil microbes known
from throughout the Precambrian,
but here a second problem surfaces -- how do you distinguish fossil archaeans
from fossil bacteria?

Archaea and Bacteria cells may be of similar sizes and shapes, so the shape
of a microbial fossil does not usually help in determining its origin.
Instead of physical features, micropaleontologists rely on chemical features.
Chemical traces of ancient organisms are called molecular fossils,
and include a wide variety of chemical substances. Ideally, a molecular fossil
should be a chemical compound that (1) is found in just one group of organisms,
(2) is not prone to chemical decay, or (3) decays into predictable and
recognizable secondary chemicals.

In the case of the Archaea, there is a very good candidate to preserve as a
molecular fossil from the cell membrane. Archeal membranes do not contain the
same lipids (oily compounds) that other organisms do; instead, their
membranes are formed from isoprene chains.
Because these particular isoprene structures are unique to archaeans, and
because they are not as prone to decomposition at high temperatures, they make
good markers for the presence of ancient Archaea.

Molecular fossils of Archaea in the form of isoprenoid residues were first
reported from the Messel oil shale of Germany (Michaelis & Albrecht, 1979).
These are Miocene desposits whose geologic
history is well known. Material from the shale was dissolved and analyzed using
a combination of chromatography and mass spectrometry. These processes work
by separating compounds by weight and other properties, and produce a "chemical
fingerprint". The fingerprint of the Messel shale included isoprene compounds
identical to those found in some archaeans. Based on the geologic history of
the Messel area, thermophiles and halophiles are not likely to have ever lived
there, so the most likely culprits to have left these chemical fingerprints
behind are archaeal methanogens (methane-producers).

Since their discovery in the Messel shales, isoprene compounds indicative of
ancient Archaea have been found in numerous other localities (Hahn & Haug,
1986), including Mesozoic, Paleozoic, and
Precambrian sediments. Their
chemical traces have even been found in sediments from the Isua district of
west Greenland, the oldest known sediments on Earth at about 3.8 billion years
old. This means that the Archaea (and life in general) appeared on Earth
within one billion years of the planet's formation, and at a time when
conditions were still quite inhospitable for life as we usually think of it.

The atmosphere of the young Earth was rich in ammonia and methane, and was
probably very hot. Such conditions, while toxic to plants and animals, can be
quite cozy for archaeans. Rather than being oddball organisms evolved to
survive in unusual conditions, the Archaea may represent remnants of
once-thriving communities that dominated the world when it was young.